The background description includes information that may be useful in understanding the present inventive subject matter. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed inventive subject matter, or that any publication specifically or implicitly referenced is prior art.
Transducers (i.e., audio loudspeakers) are well known and generally comprise a radiating surface (e.g., dome, diaphragm, membrane, cone, etc) driven by a voice coil. An electrical current is supplied to the voice coil via an amplifier, producing an electromagnetic field around the voice coil. The electromagnetic field interacts with a static magnetic field, which causes the voice coil and the radiating surface to vibrate, thus producing audio waves.
In order to improve a transducer's frequency range of audio waves, the transducer can be placed inside (or otherwise coupled with) an enclosure that has a duct (also referred to as a port). As the transducer's radiating surface vibrates, air within the enclosure is forced out of the duct, producing a sound wave at lower frequencies than the sound waves produced directly from the transducer's radiating surface. Examples of transducer enclosures with ducts can be found in U.S. Pat. No. 1,869,178. The combination of the transducer, enclosure, and duct is referred to herein as an acoustic system. Acoustic systems generally provide a larger frequency range than just the transducer alone, and enhances the listener's experience.
U.S. Pat. No. 1,869,178 and all other extrinsic materials discussed herein are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
One common problem with ducts in acoustic systems is excessive noise at high sound pressure levels (“SPL”). Since SPL is directly related to volume (e.g., loudness), poor duct designs can severely limit the acoustic performance of an acoustic system. As used herein, “acoustic performance” refers to an acoustic system's ability to produce sound waves with desirable characteristics. Desirable acoustic characteristics may differ depending on the application. Examples of desirable acoustic characterizes may include the ability to output a large frequency range of sound at high volumes with little or no noise. As used herein, the term “noise” refers generally to audio waves other than an input signal.
One primary source of noise in acoustic system ducts is the occurrence of boundary layer separation (i.e., flow separation) and vortices along the interior length of the duct and at the exit. In order to prevent boundary layer separation and vortices, acoustic system designers have historically followed the design rule of keeping the duct's air output velocity below 5% of the velocity of sound (approximately 17 m/s). See, for example, “Vented-Box Loudspeaker Systems Part II: Large-Signal Analysis,” by Richard Small (JAES Vol 21, No 6, July/August 1973). Unfortunately, this design rule leads to ducts that have larger cross-sectional areas and longer lengths for a designed resonance. For miniature acoustic systems (e.g., smart phones, tablets, flat screen displays, etc) this design rule results in unsatisfactory acoustic performance.
As an alternative approach, many designers are now providing ducts with flares (i.e., ducts that have a cross sectional areas that transition from large to small, then back to large). See, for example, U.S. Pat. Nos. 5,714,721, 5,892,183, 7,711,134, and International Patent Application Publication No. WO 90/11668. Flares help to reduce vortices at the duct exit and allow for smaller and shorter ducts than the “5% rule” for a designed resonance.
U.S. Pat. No. 5,714,721 describes another approach, in which a duct has a cross sectional profile that smoothly transitions from large-to-small-to-large. The duct's cross sectional profile is designed to expand and compress the air flow in the duct, thus reducing the air exit velocity below the recommended 5% value. U.S. Pat. No. 5,892,183 further describes a duct that has an expanding cross sectional profile of roughly seven degrees and a parabolic profile to avoid boundary layer separation. Unfortunately, these design approaches fail to fully optimize acoustic performance for any given space constraint.
U.S. Pat. No. 7,711,134 describes yet another approach, in which a duct cross sectional profile is designed as a function of its pressure gradient. More specifically, the duct is configured such that it achieves a constant pressure gradient. A similar approach is described in International Patent Application Publication No. WO 90/11668, which describes a duct that has an elliptical/hyperbola profile. While advantageous in some aspects, this approach unnecessarily limits the duct design to only those shapes and configurations that result in constant pressure gradients. More importantly, this approach fails to account for the real underlying factors that affect boundary layer separation and, like the previous approaches, fails to fully optimize acoustic performance for any given space constraint.
While these design approaches provide some improvement to previous acoustic systems, they fail to appreciate the true underlying factors that affect the performance of acoustic systems. It would be advantageous to provide an approach to duct designing that better optimizes acoustic performance within a constrained space by accounting for the underlying factors that affect the acoustic performance.
Thus there is still a need for improved duct designs and duct design rules.